The pattern of lateral-line afferents in urodeles. A horseradish-peroxidase study.

The organization of posterior and anterior afferents of the lateral-line system was studied in several species of urodeles by means of transganglionic transport of horseradish peroxidase. The afferents of each lateral-line nerve form distinct fascicles in the medullary alar plate. Each of the two branches of the anterior lateral-line nerve is organized in two long and one short fascicles. The posterior lateral-line afferents form only two long fascicles. Each ordinary neuromast is supplied by only two afferents, which run in the two ventral medullary fiber bundles. It is suggested that afferents to hair cells displaying one type of polarity form together one bundle, but those contacting hair cells polarized in the opposite way form the second ventral bundle of one lateral-line branch. Thus, the lateral-line afferents may be organized in a directotopic fashion. The short dorsal fascicle formed only by the anterior lateral-line afferents receives fibers exclusively from small pit organs. Each pit organ is supplied by only one afferent. Anatomically, these pit organs resemble in many respects the electroreceptive ampullary organs of certain fish. Neurons labeled retrogradely via the anterior lateral-line nerve afferents have been attributed to the nervus trigeminus or facialis. In addition to the posterior lateral-line afferents, only few centrifugally projecting neurons were labeled. These neurons are discussed as efferents to the posterior lateral-line neuromasts.